1. Field of the Invention
This invention relates to the calibration of single or multi-axis sensitive instruments, such as an inertial-based attitude measurement instruments or inclinometers. More specifically, the invention is a positioning system used for single or multi-axis sensitive instrument calibration and a calibration system that can be used with the positioning system.
2. Description of the Related Art
Many well-known single or multi-axis sensitive instruments, which when used typically require their orientations to be moved through known rotations about a fixed field, are commonly used. A few examples of these types of devices include a magnetometer, a gyro, an optical system, and an inertial-based attitude measurement instrument.
Inertial-based attitude measurement instruments are more commonly known as angle measurement instruments or inclinometers. One type of inertial-based angle measurement system is the quartz flexure accelerometer that is routinely used in wind-tunnel experimentation. These accelerometers can be single or multi-axis sensitive devices. The accelerometers measure static acceleration with respect to the earth's gravitational field and produce an electrical current that is proportional to their orientation relative to the earth's gravitational field reference. More specifically, the current is proportional to the sine of the angle between a plane that is normal to the earth's gravity vector and the internal axis of the accelerometer. In other words, the accelerometer is sensitive to changes in its orientation with respect to level, however it is insensitive to rotations about the earth's gravitational vector. The change in current with respect to the change in acceleration is called the sensitivity. When the accelerometer is in a level orientation, it produces a near-zero current, i.e., not exactly zero due to an internal electrical imbalance referred to as the bias.
To convert the differential current to a differential voltage for more convenient measurement by a data acquisition system, a precision resistor can be placed across the accelerometer's output terminals. Hereinafter, this electrical signal is referred to as the accelerometer's response. The accelerometer also typically includes a temperature sensor that produces a current proportional to the internal temperature of the device. In a similar manner, a resistor can be placed across the temperature sensor output terminals to provide a temperature-induced voltage response also referred to as the temperature sensor output. Due to the construction of the device, the accelerometer's bias and sensitivity vary with temperature, and therefore a calibration model of the device typically incorporates temperature compensating coefficients.
Combining three single-axis quartz flexure accelerometers forms a triaxial accelerometer system that is used in wind-tunnel experimentation to measure pitch and roll. The accelerometer can also be used for alignment and the detection of relative movement in mechanical joints. The triaxial accelerometer system incorporates three mutually orthogonal accelerometers mounted in a rigid housing. Note that the alignment in the housing is more accurately described as near-orthogonal due to fabrication imperfections of the housing and the internal misalignment of the sensitive axes of the accelerometer with respect to its external case. The triaxial accelerometer system provides simultaneous measurements of the projection of the gravitational vector onto a three-axis Cartesian coordinate system thereby enabling the prediction of sensed pitch and roll angles through trigonometric relationships. In any orientation, two angles can be determined.
The highest angular sensitivity, and thereby the most accurate angle measurement, is achieved when a sensitive axis of the (single or multi-axis) accelerometer is oriented perpendicular to the gravitational vector. In this near-level position, the accelerometer is said to be in its sensitive attitude. As the device is moved away from its sensitive attitude, there is a corresponding deterioration in the measurement resolution. For small angles, the device remains highly accurate. However, in the limiting case, the device's sensitive axis is collinear with the gravitational vector resulting in coarse predictions of angular orientation. By employing three accelerometers simultaneously, at least two of the individual devices will be less than 45° away from their sensitive attitude regardless of the orientation of the triaxial system. This maximum deviation is derived from the orientation when two of the three accelerometers sense equal components of the gravitational vector. Using this strategy, the triaxial system design performs a trade-off among the relative contribution of the individual accelerometers in determining the projected gravitational components in any orientation.
Prior to use, single or multi-axis accelerometers must be calibrated. Previous calibration systems rely on precise angular positioning of an accelerometer using a mechanically complex sequence of rotary tables mounted in a known (near-level) orientation relative to the gravitational coordinate system. While these systems represent the state-of-the-art in accelerometer calibration, they also possess certain weaknesses. For example, these systems are costly and are permanently mounted in a laboratory thereby making them incompatible for in-situ calibration. In addition, they require a complex calibration procedure to partially compensate for the near-levelness of the mounting surface and the orthogonal misalignment among the sequence of rotary tables.